I am working on the water balance modeling using WEAP in a basin which including natural Lakes. And I couldn't effectively operated the lakes in the model. can you have some experience that can share to me? Thanks in advance for all of your consideration

I would model a lake as a reservoir, I'm sure you already did that. The problem is how to constrain or operate a reservoir that mimic the behavior of a lake. So far, I know two different ways to model reservoirs successfully, I'm sure there are more. For both of them is really important to have in Key Assumptions the ACCOUNTING of the mass balance. Please do account your Inflows, Outflows and Storage following the continuity equation: Inflows - Outflows = Change in Storage.

Approach 1 :

This approach consist in 3 lines of defense
Defense line 1 - Constrain you transmission links for the water users below (or above) the lake based on the storage of the Lake. Check what is the storage at the end of the year, or in the previous month, and decide how much water you want to let it pass in the transmission link. Remember that you know the storage in the previous month or at the end of the year because you are accounting it in in Key Assumptions.Usually, the equations in the transmission link has the following for:
Min[Annual Water Use Rate User_X, Available storage Group Y* (Annual water use Rate User_X / Sum of the Annual water users rate for Group_Y) ]*Mothly_use_coeffcicient_User_X[share]/100
Defense Line 2 - Use the priorities in order to tell WEAP where does the water should come from. If you set a higher priority to the lake (lower number, let say 1) in comparison with the water users (hiogher number, let say 2), WEAP will first fill the lake and then supply water to the users. Make a conceptual map and identify which sources of water supply which water users.
Defense line 3 - Buffer Coefficient. I think the two previous lines will make all the work, but if there is some extra work to do, use the buffer coefficient, set the level of the buffer and assign how much water (in percentage) of the buffer zone (of the storage) it will be allowed to divert from the lake. This is your last defense, please do use the two previous and if neccessary, use this third one

Approach 2:

In approach one you know the demand for the water users, but you do not know what is the level of the reservoir you want to have, so Approach 2 is more oriented when you know what is the storage you would like to have in the reservoirs or lakes, many people call this, follow the "Rule Curve".For this situation, this are the steps

Step 1 - Do all your calculations (Mass Balance) to Identify what is your target storage in the Rule Curve.
Step 2 - Identify what is the proper outflow from the reservoir to meet your Rule curve. Usually, you will have constraint of Minimum and maximu outflow.
Step 3 - Set the Top of Inactive of your reservoir as the Rule Curve volume, so, you do not let any drop of water go out below the top of Inactive whic is now your objective volume. In this case, the top of inactive will be dinamic, meaning that the top of inactive will change each time step according to your "Rule Curve and Calculations.
Step 4 - Do not put any constraint in the water user below, let WEAP assign the water based in the priority.

I hope this have help, I have one tutorial of dams operations, unfortunately is in Spanish, I haven't translated to English, I will do it soon but please do not hesitate in contact me.

Subject: Re: Operating Natural Lake in WEAPPosted: 7/4/2009 Viewed: 23585 times
Hi,
I had this experience on my WEAP based projects.
Just define a reservoir. A lake is a water body which it has not outlet except evaporation.
One of the main problem for lakes is Elevation-Volume curve. Almost, for natural lake we do not have this data. But I use Google Earth for some projects. For inactive volume, my advice is a sensitivity analysis (specially if one of your goals was water quality in lake).
For top of conservation, leave it blank, because almost lakes do not have any spillway, pathway or sideway for letting current out.
If you have any data for water elevation in lake, you can comparing your real data and computed by WEAP. With above approach, I got good results.
If you have any problem, do not hesitate to contact me.

Regards,
Mehdi Mirzaee

Belete Kidanewold

Subject: Re: Operating Natural Lake in WEAPPosted: 7/5/2009 Viewed: 23561 times
Thanks both of you for your suggetions. but I want to give Hint about the lake. The lake is not closed. It has an outlet that deliver water to the river system. So don't conclude that lake doesn't have outlate and spilway.

Mehdi Mirzaee

Subject: Re: Operating Natural Lake in WEAPPosted: 7/7/2009 Viewed: 23550 times
Well,
It's easy!
You know that sometimes in Water Resources Management models, you have to define some "Dummy" nodes or links.
For your problem, you can make a division before lake or make a transmission link from lake to river or even make a "Loss to Groundwater" in reservoir menus. But you have to define a "Dummy Groundwater" in your area and then return this groundwater to the river.
All of above solution is available and applicable.